The present invention relates to a process for the production of thin semiconductor material films, preferably applicable to the production of monocrystalline films.
It is known that for producing monocrystalline semiconductor films there are various methods and processes, which are often complex and expensive to carry out, because although it is relatively easy to produce polycrystalline or amorphous material films, it is much more difficult to produce monocrystalline films.
Among the methods used for producing monocrystalline films are those used for producing socalled “silicon on insulator” substrates, where the aim is to produce a monocrystalline silicon film resting on a substrate electrically insulated from the film.
By crystal growth heteroepitaxy methods make it possible to grow an e.g. thin film silicon crystal on a monocrystalline substrate of another type, whose lattice parameter is close to that of silicon, e.g. a sapphire substrate (Al2O3) or calcium fluoride substrate (CaF2). (cf. ref. 5) (identified below).
The SIMOX process (name used in the literature) makes use of high oxygen dose ion implantation in a silicon substrate for creating in the silicon volume a silicon oxide layer separating a monocrystalline silicon film from the substrate mass (cf. ref. 1).
Other processes make use of the principle of thinning a wafer by chemical or mechanochemical abrasion. The most successful of the processes in this category also use the etch-stop principle, which makes it possible to stop the thinning of the wafer as soon as the requisite thickness is reached and in this way it is possible to ensure a uniformity of thickness. This procedure e.g. consists of p-type doping of the n-type substrate over the thickness of the film which it is wished to obtain and then chemically etching the substrate with a chemical bath active for the n-type silicon and inactive for the p-type silicon (cf. refs. 2 and 3).
The main applications of monocrystalline semiconductor films are silicon on insulator substrates, self-supporting silicon or silicon carbide membranes or diaphragms for producing X-ray lithography masks, sensors, solar cells and the production of integrated circuits with several active layers.
The various methods for producing thin monocrystalline films suffer from the disadvantages associated with the production procedures.
Heteroepitaxy methods are limited by the nature of the substrate, because the lattice parameter of the substrate is not precisely the same as that of the semiconductor, the film having numerous crystal defects. In addition, these substrates are expensive and fragile and only exist with limited dimensions.
The SIMOX method requires a very high dose ion implantation requiring a very heavy and complex implantation machine. The output of such machines is limited and it would be difficult to significantly increase it.
Thinning methods are not competitive from the uniformity and quality standpoints except when using the etch-stop principle. Unfortunately, the creation of said etch-stop makes the process complex and in certain cases can limit the use of the film. Thus, if the etch-stop is produced by p-type doping in a n-type substrate, any electronic devices produced in the film would have to adapt to the p-type nature of the films.